Interpretive Summary: The Wisconsin Phosphorus Index (WPI) is a computer model that estimates the risk of phosphorus loss in runoff from agricultural fields. It considers many factors that contribute to phosphorus loss, including soil phosphorus concentration, applied manure and fertilizer, erosion, runoff, soil type, topography, and tillage and crop management. We compared WPI estimated phosphorus loss to actual annual phosphorus loads measured in surface runoff from crop fields and pastures throughout Wisconsin. Because WPI-estimated erosion and runoff are based on average annual weather, it predicts relative, not actual, amounts of phosphorus loss from fields. But when actual soil erosion data was entered into the model, the WPI accurately estimated actual annual phosphorus loads. This shows that the WPI model is a reliable tool because it accurately predicts phosphorus loss when the unknown erosion factor is removed from the equation. This information greatly increases the validity of the WPI so it can be used with confidence when making nutrient management plans.

Technical Abstract:
The Wisconsin Phosphorus Index (WPI) is one of several P indexes in the US that uses equations to describe actual P loss processes and estimate field-scale dissolved P (DP) and particulate P (PP) loss risk in units that can be directly compared to measurements. The WPI calculations use soil P concentration; applied manure and fertilizer P; and estimates of average annual erosion, frozen soil runoff, and rainfall (non-winter) runoff based on long-term precipitation patterns, soil, topography, and planned management. We compared WPI estimated P losses to annual P loads measured in surface runoff from 86 field years on crop fields and pastures. As the erosion and runoff generated by the weather in the monitoring years varied substantially from average annual estimates, the WPI and measured loads were not well correlated. When measured runoff and erosion for each field year was used in the WPI field loss calculations, however, the WPI accurately ranked annual TP loads with a Nash-Sutcliffe Model Efficiency (NSE) of 0.87. The DP loss estimates were not as close to measured values (NSE = 0.40) as the PP loss estimates (NSE = 0.89). Some errors in estimating DP losses may be unavoidable due to uncertainties in estimating on-farm manure P application rates. The WPI is sensitive to field management that affects its erosion and runoff estimates. Provided that its methods for estimating average annual erosion and runoff are accurately reflecting the effects of management, the WPI is an accurate field-level assessment for managing runoff P losses.